I understand that you're supposed to cool stock before covering and refrigerating or freezing. What I've read is that it will sour if you cover it while it's still hot; have always wanted to taste-test that, but haven't yet.
My question is - WHY does the stock sour? What reaction happens when you cover the stock before it's cool? And will that reaction also happen if you cover stock-based soups, gumbos, etc. if you cover them while still hot?